The present invention relates generally to retroreflectors and, more particularly, to steerable retroreflectors and associated methods.
Retroreflectors are utilized in a variety of applications that require light to be reflected toward its source. For example, retroreflectors can be utilized as targets that are illuminated by a laser source such that the location of the retroreflective target can be precisely determined by detecting and analyzing the reflected light. In three-dimensional metrology, for example, applications such as laser pointing, tracking interferometry and laser radar measurement systems utilize retroreflectors as targets. In this regard, retroreflective targets can be mounted on robots, machine tools or other mechanical positioning devices, including various types of computer numerical control (CNC) devices, (hereinafter collectively referred to as xe2x80x9cmachinesxe2x80x9d) such that the position of a machine can be precisely determined by illuminating the retroreflective target and measuring the reflected light. In one advantageous example, a retroreflective target can be mounted upon the end effector of a robot such that the position of the end effector can be precisely determined.
Since the position of a retroreflective target can be precisely determined by illuminating the target, such as with a laser, and by thereafter detecting the light reflecting from the retroreflective target, retroreflective targets are advantageously employed during the manufacture of parts demanding high precision, such as parts fabricated for the aerospace and automobile industries. In this regard, retroreflective targets can be mounted upon machines, such as robots or other machine tools, utilized during the manufacture of precision parts such that the position of the machine can be precisely determined and any positional errors, such as positional errors due to changes in the temperature, misalignment or the like, can be detected and corrected. See, for example, U.S. Pat. No. 5,903,459 which issued May 11, 1999 to Thomas A. Greenwood et al. and which describes a precision measuring system and method, the contents of which are incorporated by reference herein.
While a retroreflector would ideally have an unlimited field of view such that the retroreflector could receive and reflect light that impinges upon the retroreflector from any direction, conventional retroreflectors have a limited field of view known as an acceptance angle. As such, light received by a retroreflector within the acceptance angle is reflected by the retroreflector. However, light outside of the acceptance angle is not reflected and, therefore, cannot be utilized to determine the position of the retroreflector. As such, the acceptance angle defined by a retroreflector restricts the position and orientation of the retroreflector relative to the light source. This limitation is particularly disadvantageous in applications in which the retroreflector is mounted upon a machine, such as a robot or other machine tool, that can move in multiple directions and about multiple axis relative to the light source and may frequently be positioned such that the retroreflector does not face the light source, thereby preventing the light emitted by the light source from falling within the acceptance angle defined by the retroreflector. Without adding additional light sources and/or additional retroreflectors which would, in turn, increase the cost and complexity of the precision measuring system, the position of the machine can therefore not be determined in instances in which the retroreflector does not face the light source.
One common retroreflector is a trihedral prism reflector that is frequently referred to as a solid comer cube retroreflector. The trihedral prism retroreflector has three mutually orthogonal surfaces such that light incident upon the prism is reflected generally parallel to, but laterally displaced from the incident light. While trihedral prisms are relatively inexpensive and are fairly accurate with the incident and reflected beams being parallel to within 2.0 microradians, the lateral displacement of the reflected beam from the incident beam varies due to refraction based upon the angle at which the incident light strikes the retroreflector, i.e., the incidence angle. In order to maintain accurate retroreflector properties, the trihedral prism retroreflector is therefore limited to an acceptance angle of about +/xe2x88x9215xc2x0.
Another type of retroreflector is a hollow comer cube retroreflector that is constructed of three mutually orthogonal mirrors. Although the lateral displacement between the incident and reflected beams does not vary as a function of the incidence angle, a hollow comer cube retroreflector is generally relatively difficult to manufacture and is accordingly more expensive than a comparable trihedral prism reflector. In addition, hollow corner cube retroreflectors typically have an acceptance angle of +/xe2x88x9225xc2x0.
The third type of retroreflector is a cat eye in which several hemispherical lenses are bonded to form a single optical element. While a cat eye has a larger acceptance angle, such as about +/xe2x88x9260xc2x0, a cat eye is significantly more expensive than a trihedral prism retroreflector or a hollow comer cube retroreflector. While a cat eye has a much greater acceptance angle than a trihedral prism retroreflector or a hollow comer cube retroreflector, the acceptance angle of a cat eye is still insufficient in many situations, particularly in many high precision manufacturing operations in which the retroreflector will be mounted upon the end effector of a robot or other machine tool that will assume many different positions during the manufacturing process.
One attempt to overcome the limited acceptance angles of conventional retroreflectors is to group a plurality of hollow comer cube retroreflectors in a cluster. Unfortunately, the clustered retroreflectors do not form a single, large, continuous acceptance angle. Instead, the clustered retroreflectors form a plurality of distinct acceptance angles with gaps between each acceptance angle. As such, certain angular regions still do not fall within the acceptance angle of any of the clustered retroreflectors. In addition, clustered retroreflectors have not been able to be constructed so as to simulate a single target since the retroreflectors have not been able to be positioned such that their apexes are coincident.
Accordingly, although a variety of retroreflectors are available, these conventional retroreflectors do not define acceptance angles that are sufficiently large and continuous as required by some applications. In this regard, retroreflectors that are mounted upon the end effector of a robot or other machine tool preferably have an extremely large acceptance angle since the retroreflectors will be moved through a wide range of positions during typical machining operations. As such, there remains a need for a retroreflector having a much larger acceptance angle than conventional retroreflectors, while still being capable of being economically manufactured and deployed.
A steerable retroreflective system and method is therefore provided which has a retroreflector with an extremely large effective acceptance angle, typically exceeding 320xc2x0. The area outside the acceptance is therefore a conic subtending an angle that is generally less than 40xc2x0. As such, the retroreflector of the steerable retroreflective system of the present invention can be mounted upon a machine, such as the end effector of a robot or other machine tool, and can be controllably steered such that the incident light remains within the acceptance angle of the retroreflector even as the machine is moved through a wide range of positions and orientations. Thus, the position of the machine can continue to be monitored based upon the light reflected by the retroreflector of the present invention.
According to the present invention, the retroreflector is designed to reflect the majority of the incident light while permitting some of the incident light to pass or leak therethrough. The steerable retroreflective system also preferably includes an optical detector for detecting the leakage light that passes through the retroreflector and means for controllably steering the retroreflector in response to the leakage light detected by the optical detector. The means for controllably steering the retroreflector preferably includes at least one positioner for moving the retroreflector and a controller, responsive to the optical detector, for directing the at least one positioner to controllably steer the retroreflector in response to the leakage light detected by the optical detector. For example, the controller can continuously steer the retroreflector to follow the incident light. Although the optical detector and the controller can cooperate to steer the retroreflector according to a variety of techniques, the optical detector of one embodiment defines a target zone and the controller steers the retroreflector so as to move the leakage light toward the target zone.
In advantageous embodiment, the retroreflector is a trihedral prism having an input surface through which incident light is received and a plurality of reflective surfaces for reflecting the incident light. The trihedral prism also defines an at least partially transmissive window opposite the input surface such that leakage light passes through the window and escapes from the trihedral prism. The edges of the reflective surfaces of the trihedral prism define lines that intersect at an imaginary apex of the trihedral prism. Preferably, the window is offset from the imaginary apex of the trihedral prism and is sized such that between about 0.5% and 5% of the light received through the input surface of the trihedral prism passes through the window. More preferably, the window is sized such that about 1% of the light received through the input surface of the trihedral prism passes through the window.
In operation, the retroreflector is illuminated by light incident thereupon. By detecting the leakage light that passes through the retroreflector, the retroreflector can be controllably steered, such as to lock onto and follow the incident light. By controllably steering the retroreflector, the retroreflector effectively has an extremely large acceptance angle. For example, the retroreflector can have an acceptance angle of about 320xc2x0, so as to permit light to be received and reflected by the retroreflector from a much wider range of angles than conventional retroreflectors. As a result of the significantly increased acceptance angle, the steerable retroreflective system is particularly advantageous for use with machines, such as robots and other machine tools, that are capable of assuming a variety of positions. In this regard, by having a large acceptance angle, the steerable retroreflective system of the present invention permits the retroreflector to be illuminated in almost every position that the machine assumes, while maintaining a common apex location.